Equilibrium index

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An equilibrium index of a sequence is an index into the sequence such that the sum of elements at lower indices is equal to the sum of elements at higher indices. For example, in a sequence A:

[−7, 1, 5, 2, -4, 3, 0]

3 is an equilibrium index, because:
A0 + A1 + A2 = A4 + A5 + A6

6 is also an equilibrium index, because:
A0 + A1 + A2 + A3 + A4 + A5 = 0
(sum of zero elements is zero)

7 is not an equilibrium index, because it is not a valid index of sequence A. Write a function that, given a sequence, returns its equilibrium indices (if any). Assume that the sequence cab be long.

Challenge

Print out the equilibrium indices of a given list of numbers, or -1 if there is none.

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